This invention relates to lamps and more particularly to incandescent halogen lamps. Still more particularly, it relates to a glass for the envelope of halogen lamps.
Lamps operating by a tungsten-halogen cycle are known. In operation, tungsten-halogen lamps contain a non-reactive gas filling such as neon, nitrogen, argon, krypton or xenon or a combination thereof, together with a halogen, usually bromine, which combines with the evaporated tungsten escaping from the incandescent filament. An equilibrium concentration is attained by the gaseous species within the lamp between the temperature limits defined by the incandescent filament and the coldest spot in the lamp envelope. The cold spot temperature must be sufficiently high to prevent any tungsten halide from condensing and, providing that this condition is met, a continuous transport cycle operates which keeps the envelope free from tungsten.
Numerous hard glasses, such as the aluminosilicates, have been employed with tungsten halogen lamps with varying degrees of success. Such glasses include Corning Incorporated nos. 1720, 1724 and 1725; glass nos. 8252 and 8253 available from Schott; no. 180 available from General Electric.
The 1720, 1724 and 8252 glasses have been used successfully with low voltage applications (i.e., 12 v), such as automobile headlamps which operate with wall temperatures below 500xc2x0 C.; however, for line voltage applications, i.e., voltages greater than 85 v, with wall temperatures much greater than 500xc2x0 C., these glasses prove unusable due to an inability to maintain good seals. This condition has been attributed to a structural compaction of the glass during operation. The compacted glass results in stresses, which can exceed the breaking strength of the glass, ultimately rupturing the lamp seal.
Other glasses, such as 1725, 180, and 8253, which all have substantially higher glass strain points, while employable with the higher wall temperatures generated by reduced envelope size and greater voltages, i.e., voltages in excess of 85 volts, will not fail from compaction but ultimately from blackening caused by the tungsten deposition on the inner bulb wall and, subsequently, in a non-passive lamp failure, i.e., the lamp capsule explodes. Ideally, lamp failure should be passive, i.e., by breakage of the filament. Typically, the halogen gas employed is either HBr or CH3Br. As previously noted, the concentration of the bromine (or other halogen) is critical for controlling the halogen cycle with tungsten. While the glasses enumerated above work well initially, after a given period of time there is a reaction of the alkaline earth cations with the bulb wall, thus depleting the bromine from the halogen cycle. The reaction products typically are BaBr2 and CaBr2, which show as a white haze on the interior of the lamp envelope surface. Halogen lamps are typically designed with this reaction as a limiting factor in lamp performance.
It has been suggested that the alkaline earth reaction with bromine could be decreased by the application of a barrier coating of silica to the interior surface of the lamp envelope (see U.S. Pat. No. 5,473,226, assigned to the assignee of the instant invention); however, this solution is costly and not completely effective.
It would be an advance in the art if a glass could be developed which eliminated or substantially reduced the combination effect of the halogen with the glass material in lamps using power supplies of greater than 85 volts whereby the performance (through increased lumen maintenance) and life of the lamps could be improved; that is, extended to beyond 2500 hours without substantial deterioration in light output.
It is, therefore, an object of my invention to obviate the disadvantages of the prior art.
It is another object of my invention to enhance the performance of halogen cycle lamps.
Yet another object of my invention is the provision of a glass for tungsten halogen lamps, which delivers the above results and yet remains economical to manufacture.
These objects have been accomplished, in one aspect of the invention, by providing a long life, halogen cycle, incandescent lamp for operation in excess of 85 volts which comprises: a transparent glass envelope having sealed therewithin a tungsten filament; a pair of electrical lead-ins connecting the filament and extending exteriorly of the envelope for connection to a supply voltage greater than 85 v; and a fill gas within the envelope including a halogen, at a pressure of at least three atmospheres. The envelope is constructed of an alkaline earth aluminosilicate glass having a composition consisting essentially of, in weight percent, from  greater than 58 to about 64% SiO2, from about 14 to about 17.5% Al2O3; from 0 to about 1% B2O3, from 1 to about 7% MgO, from about 5.5 to about 14% CaO, from about 6 to about 17% BaO, from 0 to about 8% SrO, and from 0 to about 1.5% ZrO2. Trace amounts of other compounds such as CeO2, or TiO2 may be present in amounts less than 1% by weight. In a preferred aspect of the invention the envelope is constructed of an aluminosilicate glass having a reduced affinity for halogens comprising, in weight percent: from 59 to about 61% SiO2, from about 15.3 to about 17.2% Al2O3; from about 0.3 to about 0.5% B2O3, from 1 to about 6.5% MgO, from about 5.9 to about 13.5% CaO, from about  greater than 6.5 to about 9.5% BaO, from 0 to about 8% SrO, from about 0.05 to about 1% ZrO2, from about 0 to about 0.3% CeO2, and from about 0 to about 0.5% TiO2.